This invention relates generally to the application of nickel-titanium alloys to medical devices and, more particularly, to medical devices made from superelastic materials, such as nickel-titanium alloy, which exhibit a differentiated degree of stiffness throughout selected portions of the structural device. Medical devices made in accordance with the present invention can be designed to be implanted either permanently or for a specific duration in a patient's body for medical treatment or can be used in the performance of a medical procedure.
Nickel-titanium alloy is a well known metal alloy possessing both shape memory and superelastic characteristics and has been used in medical device applications since this material possesses unique properties under certain conditions. Nickel-titanium alloy possesses both high elasticity and useful shape memory characteristics and is highly biocompatible, kink resistant, fatigue resistant, and has other beneficial engineering attributes which make the material particularly useful in medical applications.
The shape memory effect characteristic of nickel-titanium alloy is a result of metallurgical phase transformations. Depending on its temperature, the structural properties of nickel-titanium alloy enable it to function in two different states or phases. At the lower temperature range, below a specified transition temperature, nickel-titanium alloy loses rigidity and becomes soft and malleable and is said to be in a martensitic state or phase. However, when heated above the specified transition temperature, the nickel-titanium alloy transforms into its predetermined heat set shape and has relatively high tensile strength. In this physical state, nickel-titanium alloy is said to be in an austenitic state or phase. The shape memory properties of nickel-titanium alloy enable it to “remember” a particular shape instilled during a previous heat-set operation and to transform back to that shape when desired. Nickel-titanium alloy also can be permanently shaped by annealing with extreme heat, or by coldworking which involves plastically deforming the material.
In terms of its elasticity, nickel-titanium alloy can become highly elastic under certain conditions and is able to experience extensive deformation, yet is capable of transforming back to its original shape. Nickel-titanium alloys are known to exhibit “pseudoelastic” or “superelastic” behavior when subjected to certain cold working processes or cold working and heat treatment processes following hot working. This beneficial attribute of nickel-titanium alloy refers to the ability of the material to undergo extremely large elastic deformation, in the order of 8% or more when stressed, and to substantially fully recover all strain upon removal of the stress without adversely affecting its memory properties. Full recovery is typically understood to be less than 0.5 percent unrecovered strain, also known as permanent set or amnesia.
These characteristics allow a medical device made from nickel-titanium alloy to be deformed and restrained in a deformed condition, enabling the device to be delivered in a compressed condition into the patient's body. The deformation of the material usually causes a phase transformation to occur, e.g., austenite to martensite. Once the medical device is placed in the target area of the body, the restraint can be removed to reduce the stress on the device and allow it to return to its original pre-deformed shape within the body. This results in the material transforming back to its original austenite phase from the martinsitic phase. This phenomenon of reversible phase transformation from austenite to martensite is more precisely called “stress-induced martensite” (SIM).
Because of the useful nature of the nickel-titanium alloy, some have attempted to change its properties to solve different design needs. For example, U.S. Pat. No. 6,106,642 to DiCarlo et al. discloses annealing nickel-titanium alloy to achieve improved ductility and other mechanical properties. U.S. Pat. No. 5,876,434 to Flomenblit et al. teaches annealing and deforming nickel titanium alloy to obtain different stress-strain relationships. Some medical device related applications exploit the non-linear pseudoelastic capabilities of nickel titanium. Examples include: U.S. Pat. Nos. 4,665,906; 5,067,957; 5,190,546; and 5,597,378 to Jervis; and U.S. Pat. Nos. 5,509,923; 5,486,183; 5,632,746; 5,720,754; and 6,004,629 to Middleman, et al.
As briefly described above, superelasticity or pseudoelasticity refers to the highly exaggerated elasticity or spring-back observed in many nickel-titanium alloys deformed above their austenite finish temperature (Af) and below the martensite deformation temperature (Md). Hence, nickel-titanium alloys can deliver over fifteen times the elasticity of spring steel. The martensite deformation temperature (Md) is defined as the temperature above which martensite cannot be stress-induced. Consequently, nickel-titanium alloy remains in its austenitic phase throughout an entire deformation above Md.
In one particular medical application, nickel-titanium alloy has found use in self-expanding stents. Historically, stents were not self-expanding but rather were expanded and deployed by a balloon catheter. Balloon expanded stents are used in conjunction with balloon angioplasty procedures with the intent to reduce the likelihood of restenosis of a diseased vessel. Stents are also used to support a body lumen, tack-up a flap or dissection in a vessel, or in general where the lumen is weak to add support.
For balloon expandable stents, the stent is positioned over the balloon portion of a catheter and is expanded from a reduced delivery diameter to an enlarged deployment diameter greater than or equal to the inner diameter of the arterial wall by inflating the balloon. Stents of this type are expanded to an enlarged diameter through deformation of the stent, which then engages the vessel wall. Eventual endothelial growth of the vessel wall covers the stent.
Nickel-titanium alloy found use in self-expanding stents, where deployment was a result of either shape-memory effect or superelasticity in the material rather than by use of a dilatation balloon. The stent once released from its delivery system assumed a pre-set shape in the particular body lumen. Such self-expanding stents can be used to scaffold a number of body vessels, including the inside circumference of a tubular passage such as an esophagus, bile duct, or blood vessel.
The benefits of using a superelastic nickel-titanium material for self-expanding stents are primarily related to its large recoverable strain. The biocompatibility of nickel-titanium alloy is also an attractive benefit for use of this material in stenting applications, because the stent remains in the patient as part of the treatment.
The use of nickel-titanium alloy to form a balloon-expandable stent has been less common. The balloon-expandable and scaffolding capabilities of such stents are accomplished by setting the austenite finish temperature (Af) at about 55° C. or well above body temperature. The entire stent is therefore completely martensitic before, during, and after balloon deployment. This may be perceived as a disadvantage since the balloon-expandable nickel-titanium alloy stent remains in its martensitic phase and is somewhat soft. The scaffolding function and hoop strength of such a stent can be somewhat diminished.
This is not to say that a select portion of a stent, or other medical device, which remains in a martensitic state at body temperature is a disadvantage. Rather, certain benefits can be achieved from a stent, or other medical device, having a portion of the device remaining in a martensitic state while the remainder of the device assumes an austenitic state. For example, stents have a tendency to restenose more at their ends. This phenomenon may be prevented by making the ends of the stent, either at one end or both ends, less stiff by design or processing. In this regard, it may be beneficial if the ends of the stent are maintained in the softer martensitic state while the remainder of the stent assumes an austenitic state. Additionally, a stent or graft which is to be implanted in a body vessel having different inner diameters can be problematic to the engineer. One such example occurs when a stent is to be implanted at a bifurcated vessel in which the primary vessel has larger diameter than an adjacent side branch vessel. If the self-expanding stent exerts a uniform radial force when deployed, it is possible for the portion of the stent implanted in the smaller diameter vessel to deploy a greater outward radial force on the vessel wall than the portion of the stent implanted in the larger diameter vessel. In some instances, a larger than needed radial force on the wall of a body vessel could possibly cause unnecessary trauma to the vessel.
What has been needed in medical engineering is a medical device incorporating select portions of nickel-titanium alloy maintained in an austenitic phase with select portions maintained in a martensetic phase when the device assumes its operational state (usually implanted in the patient's body at body temperature). In this manner, such a medical device should develop variable stiffness depending on whether the portion of the device is maintained in a martensitic or austenitic phase. Once such medical device is placed in its operational state, the device should have selected portions or regions that remain stiffer than other portions or regions. Such a variable stiffness medical device would also be beneficial if it is available for medical applications including, but not limited to, stents, guide wires, guide catheters, embolic filtering devices and endovascular grafts. Moreover, it is most desirable to provide a manufacturing process by which the stiffness of various components of the medical device can be adjusted quickly and easily to suit the requirements of a particular type of application. The present invention satisfies these and other needs.